1. Field of the invention
The present invention relates to a method and apparatus for determining the defectiveness or non-defectiveness of a semiconductor laser, by automatically detecting a presence or absence of an abrupt bent portion so-called kink appearing in a waveform which is indicative of a change of differentiated optical output, which is obtained by differentiating an optical output from a defective semiconductor laser with respect to a driving current.
2. Description of the Related Art
A conventional method of determining the defectiveness or non-defectiveness of a semiconductor laser will be described below with reference to FIG. 1A to FIG. 1C.
A semiconductor laser starts light emission when a driving current (I) being gradually increased reaches a threshold current Ith of the driving current (I), as shown in a waveform 1a of FIG. 1 that is indicative of the change in the optical output (L) characteristic (to be referred to as I-L characteristic hereinafter) due to a change in the driving current (I). After the driving current (I) supplied to the semiconductor laser becomes greater than the threshold current Ith, the optical output (L) from the semiconductor laser is substantially proportional to the supplied driving current (I). Referring to the waveform 1a indicative of the I-L characteristic shown in FIG. 1A, it is determined that this semiconductor laser is defective because a bending portion 101 exists in a part of the waveform. In order to detect easily and accurately the bending portion 101 appearing in the waveform indicative of the I-L characteristic, the optical output (L) is differentiated with respect to the driving current (I) by a differentiating circuit. The optical output (L) is differentiated with respect to the driving current (I), dL/dI, and the result is designated as a differentiated optical output characteristic which reflects a change in the driving current (to be referred to as a differentiated I-L characteristic hereinafter). The waveform 1b indicative of the differentiated I-L characteristic is shown in FIG. 1B. The bending portion 101 present in the waveform 1a indicative of the I-L characteristic appears distinguishably in the waveform 1b indicative of the differentiated I-L characteristic as a concave portion or protrusion portion 1c of the waveform 1b or as a discontinuous point which is due to the combination of these portions (to be referred to a kink 1c). Referring to the waveform 1b indicative of the differentiated I-L characteristic shown in FIG. 1B, a semiconductor laser whose waveform 1b includes a kink 1c is determined to be defective. That is, if the defectiveness or non-defectiveness of a semiconductor laser is determined based on the waveform 1b indicative of the differentiated I-L characteristic, the determination can be carried out more easily, as compared to the simple determination based on the waveform 1a indicative of the I-L characteristic.
The conventional method of determining the defectiveness or non-defectiveness of a semiconductor laser using the waveform indicative of the differentiated I-L characteristic will be described with reference to FIG. 2. Such a method is described in, for example, Japanese Laid Open Patent disclosures Hei. 2-186234 and Hei. 3-227093.
A driving current (I) is generated in a current driving section 201 and supplied to a semiconductor laser 202, a differentiating circuit 203, and an A/D converting circuit 204. The semiconductor laser 202 outputs a laser light in accordance with the magnitude of the driving current (I). An optical output (L) of the laser light is converted into an electric signal by an optical output measuring circuit 205 and supplied to the differentiating circuit 203. In the differentiating circuit 203, differentiated I-L characteristic data indicative of a change in an amount of the optical output (L) in response to a change in the driving current (I) are calculated based on the driving current (I) supplied from the current driving section 201 and the optical output supplied from the optical output measuring circuit 205. In other words, the optical output (L) is differentiated with respect to the driving current (I) in the differentiating circuit 203. Further, in the A/D converting circuit 204, the differentiated I-L characteristic data and the driving current (I) are A/D converted and sent to a microcomputer 206.
The microcomputer 206 detects a point where a waveform indicative of the differentiated I-L characteristic, for example, the waveform 1b shown in FIG. 1B, rises abruptly, that is, a point where the supplied driving current (I) reaches a threshold current Ith, and estimates a line 1e (see FIG. 1C) using the point as a start point. Then, as shown in FIG. 1C, the microcomputer 206 compares the estimated line 1e with a portion of the waveform 1b indicative of the differentiated I-L characteristic in which portion the supplied driving current (I) is greater than the threshold current Ith and calculates a maximum value .DELTA.K of a distance between the estimated line and the portion of the waveform 1b. The maximum value .DELTA.K is compared with a present determination reference value. In a case that the maximum value .DELTA.K is greater than the reference value, it is determined that a kink 1c is present in the waveform 1b indicative of the differentiated I-L characteristic. As a result, the semiconductor laser 202 is determined to be defective. On the other hand, if the maximum value .DELTA.K is smaller than the reference value, it is determined that the semiconductor laser 202 is non-defective.
However, in the conventional method of determining the defectiveness or non-defectiveness of a semiconductor laser, it was impossible to detect any kink present in the vicinity of the threshold current Ith in the waveform indicative of the differentiated I-L characteristic. The semiconductor lasers can be classified into two major groups, a Fabry-Perot laser and a distributed feedback laser (to be referred to as a DFB laser). The Fabry-Perot laser oscillates in a multiaxes mode, and exhibits a plurality of wavelength components in a single optical pulse signal. On the contrary, the DFB laser is a single mode laser. Therefore, it is necessary to determine the defectiveness or non-defectiveness of a semiconductor laser based on the detection of any kink in the optical output characteristic in a wide range of the driving current (I) including a region in the vicinity of the threshold current Ith. Thus, whether or not a DFB laser is non-defective could not be determined with a high precision by the conventional method to determine the defectiveness of non-defectiveness of a semiconductor laser.
In DFB lasers which exhibit a non-defective characteristic, there is a case in which the waveforms indicative of the differentiated I-L characteristic are different from each other because of different settings of various parameters such as k (optical coupling coefficient).times.L (element resonator length), the height of a diffraction lattice, and differences between various characteristics for every wafer included in the semiconductor laser (see FIGS. 5A and 5C). Even if the different settings and above-mentioned differences are reduced to a minimum level, the waveform indicative of the differentiated I-L characteristic arbitrarily changes depending upon the shape of a diffraction lattice on the oscillation end surface of a semiconductor laser. Therefore, it is very difficult to control the waveform. Thus, in a case in which such a semiconductor laser is being tested for defectiveness or non-defectiveness, it is impossible to determine the defectiveness or non-defectiveness of the semiconductor laser based on the distance between the waveform indicative of a differentiated I-L characteristic and the estimated line. As a result, there is a lack of reliability associated with the determination of the defectiveness or non-defectiveness of a semiconductor laser.
Accordingly, using the conventional method, it is impossible to automatically determine the defectiveness or non-defectiveness of every type of semiconductor laser using numerical data of the waveform indicative of the optical output from the semiconductor laser. Therefore, in many cases, a worker looks at the waveform indicative of the differentiated I-L characteristic of a semiconductor laser to determine the defectiveness or non-defectiveness of the semiconductor laser. Accordingly, the criteria to determine the defectiveness or non-defectiveness of a semiconductor laser is different from one worker to another, resulting in an increased chance of producing low quality data from the semiconductor laser.